Itinerant ferromagnetism in the two-dimensional t-t′ Hubbard model

Using exact diagonalization techniques, the Hubbard model with hopping between nearest (t) and next-nearest neighbors (t') is studied in a two-dimensional cluster with 4x4 sites. Within a range of values of t'<0 and low densities, states with different total spin are degenerate in the noninteracting system. At a finite value of the Coulomb repulsion U,states with high spin are energetically favored and the ground state can be fully polarized. The behavior of the density of states differs from that obtained within the Hartree-Fock approximation. The Low energy region of the density of states (DOS) for the majority spins remains weakly modified by U. Our results bring further support to the idea that itinerant ferromagnetism occurs in strongly correlated systems with high noninteracting DOS at the bottom of the band.